1. Field of the Invention
The present invention relates to a fluid spring in which a fluid circulates in a sleeve and a hollow piston rod, and to a molding apparatus having the fluid spring.
2. Background Art
Generally, a side panel which is a constructional member of an automobile body is manufactured by press-molding. Usually, such a side panel is a press-molded article of a complex shape, one part of which is subjected to relatively simple molding and one part of which is subjected to relatively complicated molding. The relatively simple molding is, e.g., the molding of a curved portion having a large radius of curvature. The relatively complicated molding is, e.g., the molding of a curved portion having a small radius of curvature, or a deep drawing portion having a large depth.
Meanwhile, it is difficult to simultaneously mold a curved portion having a large radius of curvature and a curved portion having a small radius of curvature in a single process-step. Thus, in such a case, a curved portion having a large radius of curvature is first molded. Next, a curved portion having a small radius of curvature is molded. Incidentally, a molding apparatus which successively performs such molding processing is described in, e.g., JP-A-2001-001064.
That is, first, a side panel is supported by clamping an end portion thereof. In this state, e.g., a central portion of the side panel is pressed by a lower die and a movable die. Next, a part between the central portion and the end portion is pressed by the lower die and an upper die. Thus, the side panel is molded. Incidentally, the movable die is held in the upper die. With the displacement of the upper die along a vertical direction, the movable die moves closer to or away from the lower die.
More specifically, at the molding of a side panel, a clamping mechanism for clamping an end portion of the side panel abuts against the side panel. Subsequently, the upper die descends towards the lower die under the action of an urging mechanism. Accordingly, at that time, an urging force of the urging mechanism should be larger than a clamping force of the clamping mechanism. This is because of the fact that when the clamping force of the clamping mechanism is larger than the urging force of the urging mechanism, it is difficult to cause the upper die to descend. Thus, it is difficult to mold the side panel.
On the other hand, upon completion of molding of the side panel, the upper die and the movable die first ascend to expose the side panel. Next, the end portion of the side panel is released from the clamping mechanism. Accordingly, at that time, it is necessary to set the clamping force to be larger than the urging force of the urging mechanism. This is because if the opposite were the case it would be difficult to cause the upper die and the movable die to ascend.
As is understood from the foregoing description, one such a molding apparatus requires that the relationship in magnitude between the force of the clamping mechanism and that of the urging mechanism before the molding of the side pane is reversed after the molding. Regarding this, in the molding apparatus described in JP-A-2001-001064, the internal pressure of a piston of the clamping mechanism is adjusted by introducing and removing operating oil into and from an oil chamber of the piston. Consequently, the clamping force is controlled.
In a case where the molding apparatus is constructed so that operating oil can be introduced into and removed from the oil chamber of the piston, the molding apparatus inevitably requires a tank for storing oil, a pressure source (e.g., compressed air) introducing and removing operating oil into and from an oil chamber, and piping for bridging among the tank, the pressure source, and the piston. This causes problems that the size of the molding apparatus is increased, that the structure of the molding apparatus is complicated, and that a sharp rise of equipment investment occurs.